Telephony call-processing arrangements of conventional telephony switching systems implement a fixed hierarchical call-routing structure wherein the initial digits of a dialed (called) number determine selection of a route (e.g., a trunk group) or a set of routes for the call, and where subsequent digits are at best determinative of a route within the initially-selected set of routes.
As an example, consider the telephone number "538-4154". A central office switching system receiving this dialed number from a caller will normally route the call based on the first three digits "538"--over an inter-office trunk group corresponding to the office code "538" if the receiving switching system is not office 538, or over local telephone lines if the receiving office is office 538. In the former case, the receiving office will ignore the extension "4154" in its route selection; in the latter case, the receiving office will use the extension to select one of the local telephone lines. Now prepend a remote area code to that number, say 303, to produce the dialed number "303-538-4154". The receiving office will now typically route the call based on the first six digits "303-538" over an inter-office trunk group corresponding to the number "303-538" if it has such a direct inter-office trunk group available, and will route the call on the first three digits "303" over a tandem-office trunk group otherwise. In the former case, the receiving office will ignore the 4-digit extension "4154" in its route selection; in the latter case, it will ignore the 7-digit number "538-4154". Now prepend an inter-exchange carrier I.D. code to that number, say "10288", to produce the dialed number "10288-303-538-4154". The receiving office will now generally route the call based on the first five digits "10288" over a trunk group leading to the identified inter-exchange carrier, and will ignore the 10-digit number 303-538 -4154 in its route selection.
This hierarchical call-routing structure is designed into conventional call-processing arrangements of known telephony switching systems and is immutable, with few exceptions. For example, it is often possible on a conventional switching system to block all of a caller's calls to numbers having "900" as the area-code digits by routing all such calls to an intercept announcement, irrespective of what other digits, if any, may have been dialed prior to the area code. However, such exceptions are few and are limited in their scope and selectivity.
But what if, for example, it is desired that all toll calls having an inter-exchange carrier I.D. code of 10288 should route via routing pattern preference (a trunk-group specifier) 17; however, all calls to area code 303 should route via routing pattern preference 18, unless the carrier I.D. code 10333 preceded that area code, in which case the call should route via routing pattern preference 19; however, if the dialed number has 303-555-1212 as its last 10 digits then the call should always be routed to the local operator? For clarity, the requirements of this hypothetical are summarized in the following Table A.
TABLE A ______________________________________ ROUTING DIALED NUMBER PATTERN PREFERENCE ______________________________________ 10288-XXX-XXX-XXXX 17 XXXXX-303-XXX-XXXX 18 10333-303-XXX-XXXX 19 XXXXX-303-555-1212 99 ______________________________________ NOTE: X means any digit, but for the defined exceptions
Known switching systems' call-processing arrangements are not able to accommodate such a call-routing structure.
Many switching-system owners (e.g., private-party owners of PBXs, public-network owners of central office switches) find this limitation unsatisfactory. They often desire to be able to choose what is important in a dialed number in deciding how to route the call. But, with the few exceptions alluded-to above, present-day call-processing arrangements do not allow them such choice. At best, such owners are able to fund custom developments, where switching system manufacturers develop for them new call-processing arrangements that have designed into them the individual customer's present route-selection criteria. But such arrangements must be developed separately for each customer's individual criteria, and are generally no more flexible in being able to accommodate future changes to these criteria than are the standard call-processing arrangements.